Enantioselective reduction of ketones with $NaBH_4$/diglyme possibly catalyzed by trialkyl borate: optically active sec-alcohols from prochiral ketones with catalytic (-)-menthol: autocatalysis option.

Ketones can be reduced with $NaBH_4$ in diglyme without an apparent proton source, but putatively catalyzed by a trialkyl borate. This can be initially derived in situ from $NaBH_4$ and an alc., although the reaction becomes increasingly autocatalytic with time. With (-)-menthol as the initiating alc., the enantioselective redns. of a range of prochiral ketones in quant. yields and moderate ees (generally 58-87%) were realized (the autocatalysis was demonstrated in two cases; catalysis by tris-(-)-menthyl borate was demonstrated in one case). The mechanism may involve either the activation of the substrate (electrophilically) or of the hydride reagent (nucleophilically). This method offers a relatively simple and inexpensive approach to a key transformation in asymmetric synthesis